Vibroseis acquisition is based on activation of one or more Vibroseis sources. The Vibroseis sources are vibrators designed to emit a signal of varying frequency into the ground. The signal is usually referred to as “sweep” with signal starting at the low frequency are up-sweeps and those starting at the high frequency are down-sweeps.
In a seismic acquisition, single or groups of vibrators are activated in sequence. The time lag between the start of the sweep of the previous group and the start of the sweep of the subsequent group is a critical parameter. The closer the starts the more efficient use can be made of the vibrator equipment in the field. However there are limits as to how close these two start times can be, before signal caused by the second groups leaks into the recordings relating to the sweep of the first group.
In a relatively recent variant of Vibroseis acquisition, different vibrators or groups of vibrators are operated such that their respective sweeps overlap in time, i.e., one vibrator group starts sweeping without waiting for the previous group's sweep to terminate. The main benefits of these “slip-sweeps” are that significant gains in production can be achieved with the potential to double or even triple the acquisition rate. Surveys can be recorded with a higher shotpoint density, resulting in higher fold and/or denser spatial sampling for shots.
The main trade-off of slip-sweep acquisition is the reduction of the contamination caused by harmonic distortion. Recording one long continuous record containing several sweeps has an obvious implication. Traditionally, the correlation process would shift harmonics of up-sweeps to negative lag times in the record. The harmonics are then eliminated by truncating the sweep record at the start time t=0 of the sweep in question. For slip-sweep recording, however, harmonics can no longer be eliminated by truncation but contaminate preceding sweep records.
The analysis, estimation and attenuation or removal of harmonics from signals acquired through slip-sweep acquisition has been subject to numerous studies and patent applications amongst which there are:                Seriff. A. J. and Kim, W. H., 1970, The effect's of harmonic distortion in the use of vibratory surface sources, in: Geophysics, 35, 234-246 (1970);        Rozemond, H. J., Slip-sweep acquisition, in: 66th Annual International Meeting, SEG, Expanded Abstracts, 64-67 (1996);        Ras, P. et al., Harmonic distortion in slip sweep records, SEG Expanded Abstracts 18, 609-613 (1999); andthe patents and patent applications U.S. Pat. Nos. 4,159,463; 6,418,079; 6,603,707; 6,665,619; 7,050,356 and WO 2004/031806.        
With the known methods in view, the present invention proposes methods to improve the existing harmonics estimation and attenuation processes, in particular with view of further reducing the time lag between subsequent sweeps in slip-sweep acquisitions.